The IWM does not have any colourised photographs on their website. They have original colour photographs and they have original black and white photographs. Do not be confused by colourisations shared on social media which happen to leave the IWM's watermark in the bottom corner. Most crop the photograph when they do this hoping to avoid anyone realizing it's an IWM photo. Some do not.
I write this not to be argumentative but only because once something is written on the internet it's very hard to unwrite it, and later on people cannot differentiate true from false. Your statement about IWM photographs is false
Yes, the Imperial War Museum (IWM) has a collection of colorized photos of London, particularly those from the Second World War. These include rare color photographs and footage of London during the Blitz, as well as VJ Day celebrations. The IWM also has original color photos and film from the war, which they have carefully restored and colorized to bring back the original color accuracy 🧐